Abstract: The flat butt welding tests were carried out on 6061 aluminum alloy and AZ31 magnesium alloy with the thickness of 4 mm by introducing 0.05 mm Ni foil interlayer under the condition of constant travel speed and different rotation speeds using the friction stir welding (FSW) technology. The effects of rotation speeds on the distribution of Ni foil particles, the types and distribution of the intermetallic compounds (IMCs) and the strength of the joints were investigated by series of microstructure characterization and mechanical property tests. The results showed that compared to the joint without Ni, the introduction of Ni foil interlayer changed the types and distribution of IMCs in the weld nugget zone (WNZ). In WNZ, there was an obvious banded structure between magnesium alloy and aluminum alloy on which the flocculent Al₁₂Mg₁₇, granular Mg₂Ni, lamellar Al₃Mg₂, and Ni foil particles of varying sizes were distributed. With the increasing of rotation speed, the distribution of Ni became more uniform, while the Al₃Mg₂ decreased relatively, and the distribution of the brittle Al₃Mg₂ gradually changed from continuous to intermittent. The tensile strength of the joint reached the maximum value when the rotation speed was 750 r/min. Compared to the joint without Ni, the tensile strength of the joint with the introduction of Ni foil interlayer was increased by 56 MPa, which was 56.9% of the strength of magnesium alloy.